The present invention relates, in general, to reverse cycle-type refrigeration systems, such as thermal storage or ice making apparatus, having a screw compressor and an economizer cycle piping and control arrangement.
In years past, most refrigeration systems were designed to operate at fixed standard design conditions throughout the year in the interest of standardizing such systems with a fixed set of operating conditions so as to minimize the need for adjusting the settings on the various system components. For example, such designs based on fixed standard design conditions were intended to avoid having to adjust the settings on such components as the expansion valves, hand throttling valves, and the various other control and adjusting devices, as well as fixed orifice devices so as to compensate for changes in pressure differentials that might exist if changes in condensing pressure would occur.
More recently, as the cost of electrical power as escalated, the amount of potential savings that could be realized by operating larger refrigeration systems at reduced condensing pressures in ambient conditions would allow it, gained the attention of management, resulting in development of the "economizer cycle" for industrial refrigeration systems which has now become standard in the industry. The normal type of "economizer cycle" would not allow most reverse cycle-type refrigeration systems to function as satisfactory, however, because of the wide range of condensing temperatures and pressures which are encountered, as well as the great variation in heat available in the discharge gas, over the range of operating conditions of the refrigeration system. Many intricate and complicated systems of controls have been designed in an attempt to solve these problems for the various types of reverse cycle applications, but in most cases, at least as many problems have been created as were solved by such designs.
Incorporation of an "economizer cycle" in an ice harvesting-type of thermal storage refrigeration system is particularly desirable because ice harvesting-type refrigeration system applications are particularly "cost conscious," and must be justified in most cases by a relatively quick pay-back.
It is well known in refrigeration today that in industrial refrigeration systems of larger capacity, such as those over 100 tons in size, the best type of compressor to use, from the standpoint of both first cost and efficiency, is a screw-type compressor. In addition, if there is any possible use for a refrigeration load at a slightly higher temperature and pressure than the base load, then the economizer option on a screw compressor can increase both the capacity and efficiency of the screw compressor even further at no loss of base load capacity and at approximately half the brake horsepower per ton of the base load.
When using a hot gas defrost-type arrangement on an ice maker or thermal storage ice harvester to defrost the ice from the ice making surfaces, it is desirable to warm up the ice making surfaces to an optimum temperature for quick defrost and then after the ice is harvested, to pull the ice making surfaces back down to the ice making range again as quickly as possible in order to have as much time during the total cycle dedicated to making ice as possible, and a minimum time spent in harvest or reverse cycle mode in which heat is being pumped back into the ice making surfaces. In fact, for each second of time that can be subtracted from the defrost cycle, the equivalent of adding two seconds to the refrigeration mode of the cycle is achieved. To accomplish this, it is necessary to control the defrost temperature of the ice making surfaces to the optimum for fastest ice harvest in order to minimize ice meltage of the ice already made, as well as to set a maximum temperature to which the ice making surfaces that have already dropped their ice first will increase to, while the remainder of the ice is being harvested.
An object of the present invention is the provision of a reverse cycle-type refrigeration system, such as an ice maker or thermal storage ice harvester, wherein the maximum temperature that the ice making surfaces warm up to during the defrost or harvest mode is controlled by controlling the maximum pressure to which the hot gas builds up in the evaporator during harvest, this being done by relief regulator valve means set to relieve this pressure build up in the evaporator to an optimum pressure for best harvest time, and relieve the pressure regulator valve into the economizer port of the screw compressor.
Another object of the present invention is the provision of a novel piping and control arrangement for reverse cycle refrigeration systems as described in the immediately preceding paragraph, wherein the hot gas relieved by the relief regulator valve into the screw compressor accomplishes part of the load cycle at no penalty to the base capacity of the screw compressor. This enables one to control the defrost pressure during harvest and also provides most the pulled-down refrigeration capacity after harvest at no expense or load to the base capacity of the compressor at the main suction valve, thereby increasing both the capacity and efficiency of the system. An additional advantage is realized from this arrangement, in that it eliminates the sudden surge of pressure from the circuit being defrosted upon the opening of the suction stop valve, as well as the normal tendency of this sudden release of high pressure to carry slugs of liquid along with the gas which might cause "hydraulic hammer" to the piping and mechanical damage to the compressor.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.